Glaucoma ranks high among the causes of blindness, and, although the prevalence is 3 to 5% for people 40 years old or more, many patients are not conscious of the disease in the current situation since the symptom of the impaired eyesight does not appear until the last stage.
An ocular pressure test by a non-contact tonometer is one of test methods used most widely for a screening test of the glaucoma, and is important for early detection of the illness. Further, ocular pressure reduction is only the treatment which has a scientific basis in a glaucoma treatment and the ocular pressure measurement is extremely important for determining the effect of the treatment.
Currently, almost all the commercially available tonometers except a Schiotz tonometer employ a measurement method using that of a Goldmann applanation tonometer (GAT) as a standard, and calculate an intraocular pressure as “ocular pressure” by converting an external force required to applanate a part of a cornea (Non-patent document 1). Accordingly, a measurement device measuring the hardness of a cornea surface is currently distributed as the tonometer on the market.
However, actually, the true intraocular pressure and the stiffness of the cornea itself cannot be measured separately from the hardness of the cornea surface, and it has been pointed out that a conventional tonometer has a measurement error caused by an individual structural difference such as a thickness or a curvature of the cornea and an individual material difference of the cornea.
Recently, there has been proposed a measurement method to clarify the physical property of an eyeball such as the stiffness of a cornea by using a device, so called a non-contact tonometer, which measures an intraocular pressure from eyeball deformation when an air flow is sprayed to an eyeball (Patent document 1 and Patent document 2). This measurement method is also based on the measurement of the hardness of an eyeball surface and the measurement principle thereof is a regression method obtained experimentally from a clinical database, and therefore a physical amount is not clarified as an absolute value. Accordingly, this method does not have a basis of measuring a true ocular pressure. Further, another non-contact ocular pressure measurement attempts to measure the intraocular pressure from a vibration such as an acoustic wave and resonance of the eyeball (Patent document 3). Such a method from resonance phenomenon cannot discriminate each eyeball tissue such as a cornea and a sclera, and has a problem that accuracy is not sufficient because the method is considerably affected by a tissue attached to the eyeball such as an external eye muscle, an optical nerve, and an orbital fat.
While some measurement methods utilizing the resonance have been applied to a contact tonometer, the situation about the cause of a measurement error is similar to that in the non-contact type (Patent documents 4 to 6).
Meanwhile, another report about the eyeball physical property shows a numerical simulation of eyeball deformation using a finite element method (Non-patent document 2). This study shows that actual eyeball deformation can be reproduced by the numerical simulation of the eyeball deformation using the finite element method.